U.S. Pat. No. 6,445,895B2 discloses an image forming device in which a detachable guide pin is provided at a side face of belt cartridge (belt unit). A detachable guide groove for guiding the detachable guide pin and a belt tension-applying mechanism including a tension link and a spring are provided at the main unit body of the image forming device. With this configuration, when the belt cartridge is attached to the image forming device main unit along the detachable guide groove, the belt tension-applying mechanism engages with a rotating shaft of a tension roller in the belt unit, thereby applying predetermined tension or tensile force to a belt.